As fighters they will be good but not special.
They are a decent size and strength for a warrior. They don't have special fighting skills aside from maybe a really hard punch. And they'll never lose their weapons if they're made from their own bodies (shape-shifted then made rock hard).
But they keep going. And going.
- They don't tire as easily as humans, because they don't have muscle tissue that fatigues. They're clay.
- They may or may not feel pain at the point of impact (that's up to you, the author) but they won't be in constant or building pain during the battle.
- They don't bleed.
- They don't bruise.
- A hit to what in a human would be a muscle is not going to rupture anything.
- If they are cut or torn, they can repair themselves.
- If an object (arrow, dart, knife, rock) penetrates their bodies, they just spit it out.
- Poison has no effect.
- There are no internal organs to get damaged.
- Choking them doesn't kill them.
- Bones don't break.
- Tendons don't rip.
- If an arm is cut off, they pick it up with the other arm and put it back.
If a person (or, more likely, a group of people) was dedicated to destroying a golem, they could succeed. Or if enough pieces of the golem's body were cut off and unretrievable. Even if they can't continue fighting forever, they can go a lot longer than a human.
Depending on how the golem was made, "killing" it is possible, but it requires special knowledge. Spells or removing parchment from its mouth or body. You can make this easy or hard, widely known or carefully guarded.
The goylomim could, with practice, use their special skills to fight in unusual ways. Shapeshift into ropes and snares and mallets and axes (carrying blades at the ready inside them) and crossbows. But they will be most effective with ordinary fighting skills that they can do over and over without becoming tired or dead.